Ventilators of various types, including manually operated, gas powered and electrically powered ventilators are well known and are customarily used in patient circuits incorporating a closed circuit, which includes a carbon dioxide absorber and gas direction control valves, or with a non-rebreathing valve, or with an unvalved T-piece (Ayre or Bain) type circuit in which rebreathing of exhaled gases does not occur. A comprehensive selection of known types of ventilators is described in a text by W. W. Mushin et al., "Automatic Ventilation Of The Lungs," Second Edition (1969), Blackwell Scientific Publications.